Hyperloop is a conceptual mode of transportation proposed by Elon Musk, that would see relatively small pods travel through airtight tubes that have been partially evacuated of air. By traveling in a reduced pressure environment, you can achieve and maintain speeds near the speed of sound much more easily than in open air, allowing for energy-efficient and rapid transport over long distances.
Issues with modern high-speed transport
Modern high-speed transport platforms, such as airplanes and high-speed rail, are primarily speed limited by air resistance and drag. The faster you travel, the more air you run into in a given time frame. Running into this extra air acts a resisting force that attempts to slow you down. To combat the extra resistance, more and more power must be used to further increase speed, which results in the increased burning of fuel and the associated rise in emissions.
The main way to minimize air resistance is to design highly aero-dynamic shapes that allow air to smoothly flow over an object. a smooth flow of high-speed air allows for minimized air-resistance and drag effects. Aeroplanes further minimize the air resistance they face, by flying at altitudes with reduced air pressure, with less air to push out of the way, less force is required to travel at the same speed.
How hyperloop will work
Hyperloop is designed to operate in a sealed tube that has had most of the air sucked out of it. The proposed pressure that a hyperloop tube would operate at is one millibar. One millibar of pressure is roughly equivalent to one-thousandth of the air pressure at sea-level, or the air pressure at an altitude of 48 kilometres.
Note: For comparison, the highest standard cruising altitude for a 747 is 12.5 km, where the air pressure is 179 millibar.
Having reduced the primary speed limiting factor, the next issue is friction with the ground. Most ground vehicles use wheels, which produce friction and suffer from wear and tear. The main alternative to this is magnetic levitation or maglev, this works well in the train systems it has been implemented on but comes with a high cost. The alternative proposed by Musk is to use a set of air caster skis, that involves the pod floating on a cushion of air. This method should be significantly cheaper than using maglev technologies while also helping to minimize the problem of air compression in the tube.
As a pod travels along the tube that is only a little wider than it is, there isn’t much room for the air to go around the outside. This can lead to the pod essentially acting like a syringe, compressing the air in front of it more and more. By including a compressor fan on the front of the pod, the incoming air can be redirected to the air caster skis as needed and the rest pushed out of the back of the pod to help maintain speed.
Linear induction motors similar to what would be found on a maglev train or in a railgun are the proposed acceleration and deceleration method. With the minimized resistance the pods can essentially glide for the majority of their trips.
The entire hyperloop concept has been open-sourced with the idea being to encourage the engineering community to propose any and all improvements they can come up with. This should lead to a superior end product but means that the current concept may be changed before its final form.
Issues with the hyperloop concept
The main issue with hyperloop is that it requires a fully airtight tube from origin to destination. The pressure of 1 millibar is seen as a realistic and effective middle ground where a hard vacuum would just be too difficult, however, this still relies on the tube remaining airtight. There is little explanation for what would happen if a tube was damaged in situations such as a terrorist attack or an earthquake.
Even though the air pressure in the tube is low, hyperloop pods must still be designed with aerodynamics in mind. This is to avoid any potential supersonic airflows when traveling at speeds approaching the speed of sound. Even with an air pressure of just one millibar, the speed of sound is a key speed limit. To realistically travel at supersonic speeds the tube would need to be put under a vacuum.
The pod needs to transition into a standard pressure environment for both embarkation and disembarkation. This adds to the system complexity and the time required for transport.
The specifically designed SCMaglev train in japan has demonstrated a top speed of 603 km/h, roughly half that of the hyperloop proposal (1220 km/h). Without the added complexities of the pressurized tubes and with the relative ease at which the capacity of the trains can be increased, this may be a more feasible high-speed travel technology than hyperloop.